Typically, DC-to-AC power conversion from a solar-array source to the utility grid is achieved in two power converter stages to transfer the maximum available power from the solar array and supply an in-phase AC current into the utility grid. See, for example, U.S. Pat. Nos. 6,281,485 and 6,369,462; and D. K. Decker, “Method for Utilizing Maximum Power from a Solar Array,” JPL Quarterly Technical Review, 1972, Vol. 2, No. 1, pages 37–48. One power stage is controlled to track the array peak power, and the other power stage controlled to deliver the AC current to the grid. This typical configuration leads to higher component counts and possibly reduced power conversion efficiency. One benefit of the two stages of power conversion is simplicity of the control design, since the two control loops are decoupled. One control loop tracks the peak-power array voltage by controlling a first converter that is interfaced between the solar-array source and the second converter, while the other control loop produces the rectified sinusoidal current being fed from the second converter to the utility grid through a switching bridge topology that is synchronized to the utility frequency. Such decoupled pairs of control loops are described, for example, in “Sequentially-Controlled Distributed Solar-Array Power System with Maximum Power Tracking” by Kasemsan Siri and Kenneth A. Conner, presented at the Aerospace Conference, Mar. 6–12, 2004, Big Sky, Mont.
The output voltage of the first converter, which becomes the input voltage of the second converter, does not need to be so well regulated and may have significant voltage ripple depending on the size of the output filter capacitor and the operating power level. Therefore, the two cascaded-power-stage configuration provides design flexibility in choices of output voltage level and size of the output filter capacitance, as well as the input filter capacitance of the first power stage. For applications that require low operating solar-array voltage (less than 100V) for safety reasons, the two-cascaded-stage configuration provides more design freedom to choose economical power components, especially the input and output filter capacitors. In this configuration, the fundamental frequency of the solar-array AC voltage ripple can be selectively controlled to be different from (or not synchronized with) the multiples of the utility frequency. Power conversion efficiency of 85% to 90% is achieved in some cascaded-power-stage designs.
There is thus a need for further contributions and improvements to DC-to-AC power conversion technology.